1. Field of the Invention
The present invention relates to a method of heating a quartz glass tube including a quartz based glass tube. More particularly, it relates to a method of heating a quartz glass tube by using a microwave, which method is suitable for the preparation of a quartz glass optical fiber since the glass tube can be quickly heated without contamination from a heat source.
2. Description of the Related Art
In one of the methods for heating a quartz glass tube, heat is generated by the quartz glass tube itself through absorption of a microwave. In this case, although a dielectric loss of the quartz glass is very small at room temperature, it becomes larger at an elevated temperature so that the microwave can be substantially absorbed. It is reported that, as shown in FIG. 1, a quartz glass material 1 is preheated to a temperature of not less than 1,000.degree. C. with an oxyhydrogen burner 13 or the like, a preheated portion of the quartz glass material 1 is immediately introduced into a microwave applicator 12 such as a waveguide or a resonator and thereby the preheated portion absorbs the microwave (cf. J. of Lightwaves Tech., vol. LT-4, No. 10, Oct. 1986, pp. 1569).
However, it is difficult to heat sufficiently an internal portion of the quartz glass material by the method of FIG. 1 since the quartz glass has a very small thermal conductivity. Further, since an outer surface of the preheated glass material is easily cooled during transferring the preheated glass material to the microwave applicator, it is very difficult to have the quartz glass material absorb the microwave so that the quartz glass material cannot be heated sufficiently by the conventional method utilizing the usual waveguide. Since preheating with an external heating source such as the oxyhydrogen burner tends to contaminate the quartz glass, it is desired to prevent contamination in a method of heating the quartz glass, particularly for producing the optical fiber.